1. Field of the Invention
The present invention generally relates to the technique of antennas. More specifically, the present invention relates to microstrip leaky-wave antennas utilized for wideband applications for increasing the operating bandwidth of the antenna and reducing the sensitivity of the direction of the antenna major lobe with respect to the operating frequency.
2. Description of the Prior Art
A leaky-wave antenna is generally utilized for high-frequency applications, especially for millimeter waves. Compared with traditional resonant antennas, the leaky-wave antenna has such advantages as higher manufacturing tolerance, simpler shaping and easier integration with feeding system, etc. In addition to the advantages mentioned above, because a leaky-wave antenna has a characteristic that the direction of the major lobe in the radiation pattern can vary in angle as the change of the operating frequency, it also can be utilized as a frequency-scanning antenna.
In general, there are two kinds of leaky-wave antennas for generating radiated waves. The first one utilizes periodic structure. That is, the energy in this kind of leaky-wave antenna is emitted by structural periodic disturbances that cause spacial harmonics, such as dielectric gratings, metal plate gratings, and slot array on a metal slice. The second one utilizes open waveguides having the same shapes. Energy emission in this kind of leaky-wave antenna is achieved by the way in which the operation frequency of the propagation mode is assigned near to the cut-off region, such as groove waveguides, non-radiative dielectric waveguides and microstrips.
Because the microstrip line is manufactured by metal, its energy loss will much higher than that of leaky-wave antennas manufactured by high-Q(quality) dielectrics. In addition to being widely applied to various high-frequency applications, the microstrip leaky-wave antenna has various advantages, such as simple structures and easily manufacturing. Therefore, it is especially appropriate for the applications of integrated antennas and low-cost commercial antennas, etc.
FIG. 1 (PRIOR ART) is the perspective view of the conventional microstrip leaky-wave antenna. As shown in FIG. 1, microstrip leaky-wave antenna 10 is a strip of metal and placed at one side of dielectric material 20. The other side of dielectric material 20 is connected to a grounded metal plate 30. In addition, the width of microstrip leaky-wave antenna 10 is represented by W, the thickness of dielectric material 20 is represented by h, and the dielectric constant is represented by xcex5r. In general, dielectric constant is about larger than 2. The microstrip leaky-wave antenna should be operated around the cut-off region by utilizing the first higher order mode. Usually, the propagation way pertaining to the higher order modes in microstrips can be divided into four frequency regions as shown in FIG. 2 (PRIOR ART), which shows the relation between the normalized higher-order-mode phase constant (denoted by /Ko) and the normalized attenuation constant (denoted by /Ko) to the frequency (denoted by f). In FIG. 2, the phase constant of the higher order modes in the microstrips is represented by the attenuation constant of higher order modes in the microstrips is represented by, and the wave number in air is represented by Ko. The curve of the normalized higher order mode phase constant /Ko and the curve of the normalized attenuation constant /Ko in FIG. 2 are represented by numerals 1 and 2, respectively. As shown in FIG. 2, there are four regions from high frequency to low frequency.
(I) Bound Mode Region
In this region, the normalized higher order mode phase constant /Ko is larger than 1 and the normalized attenuation constant /Ko is equal to 0. More specifically, the higher order mode phase constant is larger than the phase constant of surface waves on the substrate (represented by s). That is, the energy in this region is bound in microstrip and cannot be emitted.
(II) Surface Wave Region
In this region, the normalized higher order mode phase constant /Ko is between 1 and the normalized phase constant of surface waves on the substrate (i.e., s/Ko). A tiny amount of the attenuation constant is also appeared in this region. Due to the fact that the energy carried by the microstrip leaks in the form of surface waves and cannot be emitted to the air, general antennas cannot utilize this region. Besides, the tiny amount of the attenuation constant represents the energy leakage in the form of surface waves.
(III) Space Wave Region
In this region, the normalized higher order mode phase constant /Ko is lower than 1. It means that the energy can be coupled to be the surface waves and the space waves. Due to the fact that most of the energy is coupled to the air, this region can be used to implement antennas. Besides, the attenuation constant in this region is larger than that in the surface wave region, which means the energy leakage of surface waves and space waves in physics.
(IV) Cut-off Mode Region
In this region, the attenuation constant is larger than the phase constant, which means that the cut-off feature can dominate the operation of the microstrip lines. Therefore, this region cannot be used in the applications of energy emission. Most of the fed signal energy will be reflected. Therefore, it is difficult to design appropriate antenna structures and the energy emission of such antennas is not efficient. Due to the reasons mentioned above, this region is not appropriate for antenna applications.
According to these kinds of microstrip higher order mode regions mentioned above, the microstrip leaky-wave antenna can be appropriately operated in the space wave region, more specifically, by using the first higher order mode operated near the cut-off region. The cut-off frequency of the higher modes of the microstrip can be described in details as follows. The microstrip leaky-wave antenna is different to the closed waveguide. There is no obvious separation between neighboring operation regions like the closed waveguide due to the leaked energy near the cut-off region. In fact, the propagation constant of the closed waveguide has an imaginary part (=j) in the higher frequencies at the separation point, which means that the wave can be propagated. In addition, there is a real number (=) of the propagation constant in the lower frequencies at the separation point, which means the attenuation of the propagated energy. On the contrary, there are no specific cut-off separation points for open microstrip where the higher order mode propagates on. For example, using the cavity model, the cut-off frequency of the microstrip leaky-wave antenna structure shown in FIG. 1 can be defined as:                               f          c                =                  c                      2            ⁢            W            ⁢                                          ϵ                r                                                                        (        1        )            
Wherein the light speed is represented by c, the width of microstrip 10 is represented by w, and the relative permittivity of dielectric material 20 is represented by r. Next, the frequency bandwidth is described as follows. As described above, the space wave mode is the most appropriate one for antenna applications and the normalized phase constant is between 1 and the cut-off points. Using this relation, the radiation bandwidth can be deduced as:                               f          c                 less than         f         less than                                             f              c                        ⁢                                          ϵ                r                                                                                        ϵ                r                            -              1                                                          (        2        )            
As mentioned above, the dielectric constant of the substrate is usually larger than 2. The maximum usable bandwidth of the traditional microstrip leaky-wave antennas, according to the frequency bandwidth defined in equation (2), is about 40%. The usable bandwidth in practical applications usually cannot reach even 20% while considering other factors such as the bandwidth of the feeding system, the limitation of the antenna size (length) and the antenna gain etc.
Besides, the characteristic that the direction of major lobe will be varied in angle as the change of the operating frequency can be described by using the equation below. In other words, by the concept of whether phase angle is matched, the angle of the major lobe of the antenna can be roughly determined as the equation below:                     θ        =                              cos                          -              1                                ⁢                      β                          k              0                                                          (        3        )            
The propagation constant can change as the varying operating frequency. According to equation (3), the angle of the major lobe also changes during using the antenna. These kinds of antennas can be utilized for applications of phase array antennas by utilizing the characteristic above. That is, one scanning dimension is controlled by utilizing conventional phase shifters and the other scanning dimension is controlled by utilizing the change of the operating frequency. Therefore, phase shifters originally used in the one-dimensional control mechanism for these phase array antennas can be waived. Utilizing the microstrip leaky-wave antenna to manufacture a phase array antenna is low-cost due to the reduction of the expensive phase shifters. On the other hand, high-gain antennas, or called the point-to-point satellite receiver antennas, can also be manufactured by utilizing the microstrip leaky-wave antenna. However, the shift of the main lobe in this kind of antennas will cause a problem in their application. More specifically, if these antennas are applied to the narrow bandwidth applications, such as around 1% of the bandwidth, the shift of the main lobe is quite small. However, if these antennas are applied to wide-band applications, such as larger than 10% of the bandwidth, the shift amount of the main lobe is huge based on equation (3). It will cause such problems as disturbance or the degradation of the system quality for the point-to-point communication.
According to the reasons mentioned above, the microstrip leaky-wave antenna could be easily applied for some specific applications, but not appropriate for some other applications due to their characteristics. According to equation (2), for example, the bandwidth of the microstrip leaky-wave antenna is narrow, which makes it difficult to be applied for wideband applications. In addition, according to equation (3), the direction of the major lobe is sensitive to the operating frequency, which can facilitate the applications of the array antennas, but is not appropriate for point-to-point communications.
Therefore, an object of the present invention is to provide a wideband microstrip leaky-wave antenna, which has an increased operating bandwidth and a low sensitivity with respect to the operating frequency.
The present invention achieves the above-indicated objects by providing a first type of the wideband microstrip leaky-wave antenna. It comprises a substrate constituted by at least one dielectric layer and having a cavity, a microstrip line made of conductive material and located on a first surface of the substrate corresponding to a location of the cavity, and a conductive plate made of conductive material and located on a second surface of the substrate opposite to the first surface. This structure can reduce the effective dielectric constant. As described above, when the effective dielectric constant of the substrate is reduced, its corresponding bandwidth will increase.
The present invention discloses a second type of the wideband microstrip leaky-wave antenna. It comprises a substrate constituted by at least one dielectric layer and having a cavity, a microstrip line made of conductive material and located in the cavity of the substrate, for emitting leaky waves, and a conductive plate made of conductive material and located on a surface of the substrate, the dielectric layer of the substrate being excluded from a space between the conductive plate and the microstrip line. Since the space between the microstrip line and the conductive plate does not include the dielectric material and is filled up with the air, the effective dielectric constant is almost 1. When the effective dielectric constant approaches 1, the corresponding bandwidth will increase.
In addition, the present invention also discloses a third type of the wideband microstrip leaky-wave antenna. It comprises a substrate constituted by at least one dielectric layer and a microstrip line made of conductive material and located on a first surface of the substrate, for emitting leaky waves, wherein on a second surface of the substrate opposite to the first surface there is no conductive plate corresponding to the microstrip line. This type of antennas can provide an enlarged bandwidth. In addition, the normalized phase constant xcex2/k0 is almost constant within the range of the bandwidth. As described above, the direction of the major lobe emitted by this type of antennas is insensitive to the operating frequency. In addition, the substrate portion in this kind of antennas also can be omitted and, therefore, the microstrip line is surrounded by the air. The energy source can be fed into an end of the microstrip line. This simplified structure also can achieve the object of the present invention.